1. Field of the invention
The invention concerns data transmission, in the telecommunications field, in accordance with the ISO standard protocol, and more particularly in accordance with levels 1 and 2 of the standard.
2. Description of the prior art
The specific implementation behind the development of the invention concerns an HDLC (High-level Data Link Control) transmitter with 32 channels multiplexed onto a PCM link and integrated into a PCM controller associated with a data switch, for example.
However, the invention encompasses other implementations in which an ISO level 2 frame format (in place of the HDLC format) is combined with multiplexing of multiple formatted channels onto the transmission link (in place of the PCM technique).
One example of the use of PCM multiplexing of HDLC frames is the TRANSPAC (registered trademark) network which uses the X.25 protocol.
HDLC encoding entails formatting the data into successive identifiable frames each comprising a frame validation symbol on two bytes (signature based on the bits of the frame) that is checked at the receiving end.
The PCM transmission technique time-division multiplexes N independent logical channels onto a single physical transmission pair in the form of PCM frames each identified by a PCM frame start/end byte. Within each PCM frame each channel is reserved the same byte of predetermined rank.
The insertion of the HDLC frames into the PCM format at the transmitting end and then the recovery of each channel at the receiving end presupposes the provision of a specific system at each end of the transmission system. The invention concerns the transmit part of a system of this kind.
There are already known systems for transmitting HDLC frames on PCM type channels comprising 16 or 32 separate transmitters or multiple automatic devices each associated with 16 or 32 words of RAM. In the known system shown in FIG. 4 the HDLC formatting is done, on a per-channel basis, by means of a specific line for each of the channels comprising a dedicated HDLC circuit 41 and a dedicated processor 42 associated with a buffer memory 43. Each of the lines 44 corresponding to a separate channel feeds a common multiplexer 45 which constructs the PCM frame.
This existing system is fully operational but has the disadvantages of a large number of components (one component for each channel) and the resulting complexity of management.
These disadvantages are particularly constricting in developing switching systems for a very large number of lines carrying large amounts of digital data. Until recently, 32-channel PCM links carried a limited number of logical channels (two, for example), the other channels being speech channels. It was therefore feasible, and sometimes essential, to process each channel separately, the multiplication of the components 41, 42, 43 on just a few parallel channels being compensated by the resulting flexibility of configuration.
There are now being developed PCM type transmit/receive systems comprising only digital channels. For example, the signalling transfer points (STP) designed to be installed on the French public switched telephone network require a processing capacity in the order of 500 64 kbit/s HDLC channels.
The increasing digitization of the network and rising data signalling rates are now making it possible to introduce services offering increasingly superior performance (ISDN) and represent a clear requirement for better performance PCM/HDLC systems.
An objective of the invention is to provide a system which can meet these requirements whilst being economical in terms in the number of components used, and in particular the number of HDLC formatting components. The system in accordance with the invention also makes it possible to save on the use of a data multiplexer on the PCM link.
The invention also makes it possible to make the system much more compact with reduced unit cost and power consumption. What is more, the interface to the next higher level is simplified.
The invention also makes it possible to operate at a high speed with no risk of starving the PCM link.
Further, the design of the system makes it compatible with protocols differing from the HDLC format, simply by replacing a single circuit per 16-channel or 32-channel transmitter.